Tomatoes seed very easily, and since they are a commonly eaten food, their seeds often escape. In gardens, squirrels and other animals disperse the seeds when they steal the fruits and suck the moisture out of them. They are also dispersed with people's trash. A dropped slice of tomato will spread viable seeds which will come up in small plots of dirt and even sidewalk cracks. Yes, it may be noted that the edible tomatoes are a fruit, actually even a berry.

The tender of the above plot pointed out that the plant was a volunteer there. I had assumed it was planted until I was told otherwise.